1. Field of the Invention
The invention relates to circuitry to testers for testing digital circuits and isolating faults therein.
2. Description of the Prior Art:
U.S. Pat. No. 4,194,113 describes in detail a system for testing digital circuits and isolating faults therein using a "guided probe" approach. As testers in which the described system is embodied have become increasingly popular, it has become desirable to implement lower cost versions of such testers, referred to herein as "field maintenance processors". A field maintenance processor includes a general purpose microcomputer which can implement the guided probe testing operations and can also be used for various other purposes. However, interfacing a general purpose microcomputer to a wide variety of digital systems which need to be "field tested" requires construction of suitable, "custom designed" circuits referred to as "device specific adaptors" to facilitate connection of the digital circuits under test to the field maintenance processor. A digital circuit under test may be a complex, computer-based product which can "take control" of an internal data bus and/or address bus of the field maintenance processor. The device specific adaptors need to have "pluggable connectors" which facilitate physical connection of device specific adaptors to the field maintenance processor.
If the device specific adaptor takes control of the "intenal field maintenance processor (FMP) bus", it is necessary that this be done in such a way as to avoid any loss of information stored in the dynamic memory of the field maintenance processor.
Accordingly, it is an object of the invention to provide a system and method for avoiding the effects of undetected loss of information in a field maintenance processor when a device specific adaptor connected thereto takes control of a bus in the field maintenance processor.
It is very important to the manufacturer of a field maintenance processor that it have extremely high operating reliability, so that users of the field maintenance processor will never have any reason to doubt the results of a particular test. However, buyers of field maintenance processors frequently will design their own device specific adaptors for interfacing between their products to be "field tested" and the field maintenance processor. It is expected that circuit designers who "custom design" their own device specific adaptors will occasionally ignore various timing specifications (which usually are quite conservative) of the field maintenance processor. In the event that a "customer designed" device specific adaptor asserts control over the FMP bus for too long a period of time, this may result in unexplained malfunctions resulting from loss of data stored in the dynamic memory of the field maintenance processor because of the inability of the field maintenance processor to quickly refresh dynamic memories while the device specific adaptor is in control of the internal FMP bus.
Therefore, it is another object of the invention to provide a system and method for preventing a device specific adaptor from taking control of an internal bus of the field maintenance processor for a time period that is so long that the integrity of data stored in dynamic random access memories contained in the field maintenance processor is jeopardized.